1. Field of the Invention
This invention relates to a metal fusion bonding and in particular to a process and product where copper based fins are joined on copper based heat exchanger tubing using a high tin solder.
2. Description of Related Art
Finned heat exchanger tubing has been used for a variety of gas to liquid heat exchangers. As used herein the phrase "gas" includes all elastic fluids, including, for example, vapors. In addition, the term "copper-based" includes copper as well as copper alloys which contain at least 50% copper. In the power generation industry, finned copper based heat exchanger tubing is used in, for example, hydrogen coolers, exciter air coolers and oil coolers.
The conventional method of attaching copper fins to copper tubing utilizes tin-lead solder and can be either a flow solder or a dip solder type process. While high tin solder has been used for hand soldering of copper-based parts, such a solder has a very limited application on a production basis. During dip-soldering of copper-based material with high tin solders, the solder in the reservoir quickly picks up about 2% copper (depending on temperature). As this solder further reacts with the copper based surfaces the copper and tin form intermetallic compounds, most notably the "eta phase" compound Cu.sub.6 Sn.sub.5. The eta phase, having a higher melting point than the solder, will precipitate as a "sludge" which builds up and eventually renders the solder unusable. Such high tin processes have been dipped soldering processes rather than flow soldering due to the difficulty of pumping a sludge containing solder. High tin solder reservoir reprocessing on-line to eliminate the eta phase from the reservoir is generally impractical as the eta phase sludge is difficult, if not impossible, to pump into a reprocessing system. Because of sludge buildup problem, such high tin soldering is generally too expensive to do even with a dip process due to the expense of either replacing or reprocessing solder. As used herein, the phrase "high tin" means containing at least 75% tin.
As a result of the problems with high tin soldering, heat exchangers such as a hydrogen cooler (which is used to cool hydrogen gas which is in turn used to cool the generator in an electric power plant) have used tin lead (e.g. 20 tin-80 lead) solder to solder copper fins on copper based tubing. It has been discovered, however, that impurities in the system can react with the lead in the solder to form lead carbonate which can short out electrical windings of the generator.